Among the materials that impart detergency to lubricating oils to keep internal engine parts clean and reduce sludge formation in the oil are overbased detergents, for example, calcium sulfonates. The term “overbased” is a term of art which is generic to well known classes of metal salts or complexes. These materials have also been referred to as “basic,” “superbased,” “hyperbased,” “high-metal containing salts,” and the like. Overbased compositions are metal salts or complexes characterized by a metal content in excess of that which would be present according to the stoichiometry of the metal and the particular acidic organic compound, e.g., a sulfonic acid, reacted with the metal.
Overbased calcium sulfonates are generally produced by carbonating a mixture of a disperse medium, a sulfonic acid, calcium oxide or calcium hydroxide, and promoters such as methanol and water. The calcium oxide or hydroxide reacts with the gaseous carbon dioxide to form calcium carbonate. The sulfonic acid is neutralized with an excess of CaO or Ca(OH)2 to form the sulfonate. The calcium carbonate may be either amorphous and/or in one or more of its crystalline forms. The common crystalline forms of calcium carbonate are vaterite (hexagonal), calcite (hexagonal-rhombohedral), and aragonite (orthorhombic). Calcite is the most thermodynamically stable form. Both vaterite and aragonite are metastable at normal temperatures. Aragonite is not commonly observed in overbased calcium sulfonates. For lubricating oil compositions such as engine oils, the calcium carbonate that is used is typically amorphous.
Overbased calcium sulfonate detergents containing crystalline calcium carbonate disclosed in the prior art are hazy and exhibit poor oil solubility. These detergents tend to have relatively high viscosities of about 1000 cSt or higher at 100° C. Overbased calcium sulfonates of this type are disclosed, for example, in U.S. Pat. Nos. 3,242,079; 3,376,222 and 4,560,489. These sulfonates are described as being useful as additives for greases, paints (for rheology control) and metal working fluids (as extreme pressure (EP) agents).
The lubricating oil art, particularly as directed to automotive crankcase and other engine oils, mandates a clear or substantially haze free product for requisite consumer aesthetics and acceptance. This need has precluded the use of prior art detergents with haze producing crystalline calcium carbonate.
For more than 40 years, zinc dialkyl dithiophosphates (ZDDP) have been used as antiwear additives in engine lubricating oil compositions. During this period typical concentrations of phosphorus contributed to the lubricating oil compositions by these compounds exceeded 0.10% by weight. However, ILSAC GF-3 requirements subsequently came into effect and these requirements limit the amount of phosphorus that can be used in a lubricating oil composition to a maximum concentration of 0.10% by weight. There are also marketing reasons for requiring lower phosphorus levels, for example, phosphorus levels up to about 0.08% by weight, due to the fact that phosphorus tends to reduce emission catalyst life and the desire in the marketplace to prolong such catalyst life. This has resulted in the need for reformulating many engine lubricating oil compositions to provide such engine lubricating oil compositions with lower levels of phosphorus and yet still maintain adequate levels of antiwear protection.
The problem is to provide low-phosphorus engine lubricating oil compositions with desired anti-wear properties and at the same time provide haze levels that are sufficiently low to be commercially acceptable. In at least one embodiment of the invention, the overbased vaterite containing salt compositions of the present invention provide a solution to this problem. These overbased vaterite containing salt compositions, in at least one embodiment, are substantially haze free and provide enhanced antiwear properties to engine lubricating oil compositions. These overbased vaterite containing salt compositions may also be advantageously employed as additives in functional fluids such as metal working fluids.